The invention relates to an adjustable supporting device for a drilling rod. The adjustable supporting device comprises supporting assemblies which are symmetrically arranged and supported at the two ends of the drilling rod. An interval adjusting rod is mounted between the two supporting assemblies. Each supporting assembly comprises a base, a supporting seat mounted in a transverse sliding groove in the upper surface of the base and an adjusting stand column which is located on one side of the supporting seat and fixedly connected with the base. A supporting plate is rotationally mounted at the upper end of each base through a hinge shaft. Each supporting plate is connected with the corresponding adjusting stand column through a pull rod. One end of each pull rod is hinged to the side face of the corresponding supporting plate, and the other end of each pull rod is provided with a vertical groove hole which is provided with a thread and penetrates the corresponding adjusting stand column. Adjusting nuts are spirally mounted on the portions, located on the two sides of the corresponding adjusting stand column, of each pull rod. Adjusting handles are mounted on the adjusting nuts located on the outer side. The interval adjusting rod comprises three sections, namely the two outer sections connected with the supporting seats and the middle section connected between the two outer sections, and adjusting jackets are mounted between the middle section and the outer sections.